December 2009 Electrical Apparatus

December 2009 Electrical Apparatus

This is a summary of the Electrical Apparatus December 2009 featured technical article,  by Richard L. Nailen, P.E.    

Like many other manufactured products, electric motors have undergone size and weight reductions as operating experience and materials development have reduced uncertainties in design. Often, however, users complain that machines don’t seem to last as long or carry as much overload as they once did. “Not made like they used to be” is a typical objection to the newest product. Objectors may advocate lower motor winding temperature, larger bearings, and stronger shafts. Larger lamination diameters, longer core stacks, and more copper in windings have also been proposed as more “conservative” design practices.

Unfortunately, using “extra” materials–either in electrical insulation or mechanical construction–inevitably adds cost. When extra materials can be shown to achieve specific savings in product maintenance, operating expense, reliability, or longevity, the cost may be justified. However, such a decision can be based only on a statistically valid sampling of otherwise identical machines operating under identical conditions. Such a comparison is almost never possible.

Typically cited as evidence of “less conservative” motor design is thinner insulation, operating at higher temperature. Supposedly, “Cooler motors last longer.” But a properly insulated Class F winding offers at least as great a life span at its rated temperature as a Class A winding does at its lower temperature. Furthermore, motor failures occur far more often, and sooner, for reasons other than thermal aging.

Users often prefer larger bearings as evidence of conservative design. But the larger the bearing, the higher its friction loss and internal temperature. If the standard, “smaller” bearing in a direct-coupled motor has an expected life of centuries, doubling the figure by using the next larger size offers no practical benefit. Most bearing failures result from contamination, overgreasing, and misalignment; bearing size is not the governing condition. Users may complain that a modern motor’s standard structural components, such as the frame, are too lightweight. More massive shapes are thought to offer longer, more reliable service. But all structural design aims at the most cost-effective balance between strength, stiffness, durability, and cost. Cast iron, though offering high corrosion resistance, is brittle and heavy. Steel fabrication is stronger and avoids the limitations of casting molten metal, but is costly.

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